1. Field of the Invention
The present invention concerns a device integrating a pressure sensor for measuring pressures in an internal combustion engine.
2. Description of the Related Art
In an internal combustion engine, in particular a Diesel engine, each cylinder typically has a glow plug for heating the interior of the corresponding combustion chamber, in particular when starting the engine. This glow plug is located in a threaded bore that passes through the cylinder head of the engine. This glow plug therefore includes a threaded body adapted to be fitted into the corresponding bore of the cylinder head and a finger in which a preheating electrode is housed.
It is also known to integrate a pressure sensor into such a glow plug. Indeed, it has been noticed that a knowledge of the value of the pressure inside each cylinder made it possible to improve control of the combustion in the engine. This information is then used to regulate the injection of fuel into each of the cylinders. In this way, pollutant emissions from the engine can be reduced and fuel consumption optimized.
In prior art glow plugs integrating a pressure sensor, the latter is often housed in a portion of the body of the glow plug called the glow plug head or mounted on the glow plug head. The latter is the portion of the body of the glow plug that is outside the cylinder. The document EP-1 096 141 discloses one such glow plug, for example.
In these glow plugs, it is necessary to transmit the pressure in the corresponding cylinder in the engine to the pressure sensor located outside the engine, in or on the glow plug head. Various assemblies have been described such that the forces exerted on the finger of the glow plug situated in the cylinder are passed on to the pressure sensor. Various mechanical parts are then inserted between the pressure sensor and the finger of the glow plug. These various intermediary parts influence the measurement made.
The document FR-2 884 299 proposes a glow plug including a tubular body with a glow plug head and a fixing zone for fixing it into a bore, a finger mounted on the body of the glow plug at the opposite end to the glow plug head, and a pressure sensor. In such glow plugs, the finger is fixed to the glow plug body so as to be fastened thereto in a connecting zone, and the glow plug body has, between its connecting zone connected to the finger and its zone for fixing it into a bore, an elastically deformable part such that said connecting zone is movable and can be moved in the longitudinally relative to the fixing zone in a bore that is assumed to be fixed. The pressure sensor is located between a member fastened to the connecting zone and a fixed member of the glow plug.
In this way, the elastically deformable part acts as a membrane that divides the body of the glow plug into two parts, a fixed part intended to be mounted in a cylinder head and a movable part subjected to the pressure in a cylinder of the corresponding engine. This membrane can be deformed and the movable part can be moved longitudinally. That movement, which depends on the pressure in the cylinder is then transmitted to the pressure sensor, which can thus give an indication of the pressure exerted on the finger of the glow plug. With this kind of glow plug, the measurement from the pressure sensor is no longer disturbed by unwanted vibrations. Indeed, the movement of the membrane is not affected by the stresses in the cylinder head or in the rest of the body of the glow plug.
The documents DE-10 2006 008 639 and DE-10 2006 059 693 show glow plugs integrating a pressure sensor. In these plugs, the pressure sensor is placed between a fixed part, fastened to the body of the plug, and disposed above the sensor (that is to say at the opposite end to the finger), and the finger transmits, via insulating parts, the exterior pressure to which it is subjected. A part, named elastic membrane, connects the finger to the exterior of the plug to dissipate the heat and not influence the pressure measurements. The function of the elastic membrane described in each of these documents is entirely different from that described in the document FR 2 884 299.
It is also known, in Diesel engines (generally large cubic capacity engines), and petrol engines, to have a dedicated device for measuring the pressure in the combustion chambers of the engine. Such a of device then includes a body having means for fixing it into a cylinder head and inside which a pressure sensor is located. The present invention also relates to such pressure measuring devices. In the remainder of the description, when plugs or glow plugs are referred to, pressure measuring devices are also concerned, which are sometimes known as SAPS (Stand Alone Pressure Sensors), but for simplicity these are not explicitly mentioned.
A glow plug is fitted, as indicated above, by screwing the glow plug into a threaded bore. To provide a seal, an abutment is provided at the end of screwing. This abutment is conical and receives a bearing cone made on the body of the glow plug. When tightening the glow plug, the bearing cone comes up against the conical abutment of the cylinder head and a tightening torque is exerted to ensure a good seal between the glow plug body and the cylinder head. Because of this tightening torque, reaction forces are induced at the surface of contact between the glow plug body and the cylinder head. These forces are normal to the conical contact surface and therefore have a longitudinal component and a radial component relative to the glow plug body.
In the case of a glow plug as described in the aforementioned document FR-2 884 299, the bearing cone is in the immediate vicinity of the elastically deformable portion, also referred to as a membrane. Because of this, the forces induced by the tightening torque create a torque acting on the membrane that impacts on the prestressing exerted on the pressure sensor of the glow plug described. As a function of the tightening torque, the torque acting on the membrane varies and thus likewise the prestressing of the pressure sensor. Thus, if a glow plug is removed and then refitted, the tightening torque being produced by the torque wrench, there should not be any significant impact on the prestressing of the pressure sensor. However, the forces at the bearing surface do not depend only on the tightening torque but also on the conditions under which this tightening is effected: state of the threads of the glow plug and in the cylinder head, presence or absence of lubricant, etc. For the same tightening torque, forces can be induced at the bearing surface of the glow plug on the conical bearing surface of the cylinder head that vary by a factor of from one to two.
The above remarks are equally applicable to a stand-alone pressure sensor produced according to the teaching of the aforementioned French document.